Structural moment detectors, which are basically autocollimators which are insensitive to linear dynamic motion but which respond to angular deflection of one end of the sensor with respect to the other, are known in the art. For example, such sensors are disclosed in the patent to Rossire, U.S. Pat. No. 3,229,511 and in the publication entitled "The Structural Rigidity Sensor: Applications in Non-Destructive Testing", published by the Air Force Systems Command, United States Air Force (Frank J. Seiler, Research Laboratory, Publication SRL-TR-75-0017, October 1975). See also the U.S. Pat. to Okubo Nos. 4,159,422 issued June 26, 1979 and 4,164,149 issued Aug. 14, 1979.
Systems which employ structural moment detectors to measure and record certain effects of forces acting on a structure are also disclosed in the publications described above. For example, the Rossire patent discloses an aircraft attitude control system in which a structural moment detector is used to sense wing loading and automatically adjust the attitude of the aircraft to maintain wing loading within safe operational limits. The Air Force publication and the Okubo patents disclose systems which employ structural moment detectors to obtain the "vibration signatures" of various structures such as airframes, buildings, aerospace vehicles, rotating machinery bearings, dams, and the like.